Pressing device

ABSTRACT

A pressing device has pressing elements which are hollow and closed, each having first and second heat transmission walls with a heat transporting medium present in the cavity within each element. The medium absorbs thermal energy from the first heat transmission wall while changing from the liquid phase into the vapor phase and delivers thermal energy to the second heat transmission wall, while changing from the vapor phase into the liquid phase, and absorbs thermal energy from the second heat transmission wall while changing from the liquid phase into the vapor phase and delivers thermal energy to the first heat transmission wall while changing from the vapor phase into the liquid phase. In each cavity a porous mass is present connecting the heat transmission walls in such manner that all the heat transporting medium condensed on the second or first heat transmission wall can flow respectively through the said mass by capillary action.

Dirne [4 1 Sept. 25, 1973 PRESSING DEVICE [75] Inventor: Andrianus Petrus Dirne,

Emmasingel, Eindhoven, Netherlands [73] Assignee: U.S. Philips Corporation, New York,

[22] Filed: Sept. 7, 1971 [21] Appl. No.: 177,944

[30] Foreign Application Priority Data Sept. l0, 1970 Netherlands 7013364 [52 us. on 425/407, 65/356, 425/408,

' 425/78 [51] Int. Cl. B29c 3/00 [58] Field of Search ..'425/l74, 384, 233, 425/363, 352, 354, 407, 406, .408, 78; 65/103, 356

[56] References Cited UNITED STATES PATENTS 3.178.792 4/1965 I Scott 425/407 X 3,644,110 2/1972 Sendt....' 65/356 X Primary ExaminerRobert L. Spicer, Jr. Att0rneyFrank R. Trifari 57 ABSTRACT A pressing device has pressing elements which are hollow and closed, each having first and second heat transmission walls with a heat transporting medium present in the cavity within each element. The medium absorbs thermal energy from the first heat transmission wall while changing from the liquid phase into the vapor phase and delivers thermal energy to the second heat transmission wall, while changing from the vapor phase into the liquid phase, and absorbs thermal energy from the second heat transmission wall while changing from the liquid phase into the vapor phase and delivers thermal energy to the first heat transmission wall while changing from the vapor phase into the liquid phase. In each cavity a porous mass is present connecting the heattransmission walls in such manner that all the heat transporting medium condensed on the second or first heat transmission wall can flow respectively through the said mass by capillaryaction.

8 Claims, 7 Drawing Figures PATENIED SHEEYIBF'I INVENTOR. ADRIAN us P. DIRNE QJVGLZZU PATENTED $929973 SHEEY 2 BF 7 "EIE'AYINMENTOR. ADRIANUS P. mama BY MJflZJit PATENTEI] SEPZBIQH NEW 0? 7 INVENTOR. ADRIANUS P. DIRNE ZZMKJ M Agent PATENTEUSEPZSIBH sum w 7 VEN'l'bR.

ADR IANUS F. D E

Agent PATENTED SEPZS I975 SHEET 5 BF 7 INVENTOR. ADRIANUS P. m RNE BY wt PAENTEUSPZSW mm EFT INVENTOR. ADRIANUS R DIRNE Agent INVENTOR.

Agent ADRIANUS P. DlRNE PATENTE0 2 DOM/3 PRESSING DEVICE The invention relates to a pressing device for pressing materials to solid bodies. The device comprises at least two pressing elements which are relatively movable in the direction of pressing and have pressing surfaces which face each other and can cooperate and which bound a pressing space which is accessible for the material to be pressed. Thermal energy is supplied to and withdrawn from, respectively, the pressing surfaces during operation.

Devices of this type are known and are used for all kinds of purposes for example, for pressing glass articles, such as lenses and prisms, (color) television display screens, cups and cover glasses of pressed glass lamps, for manufacturing articles of synthetic resins, for densely sintering powders of ceramic materials, and so on.

The pressing elements usually consist of dies (see, for example, German Patent Specification No. 397,427, Dutch Patent Specification No. 1 1,410), of matrices or of a combination of a die and a matrix (German Patent Specification No. 1,292,803; published German Application No. 1,924,391).

In accordance with the use, thermal energy is supplied to one or both pressing elements during operation of the pressing device, usually by means of electric resistance wire heating or by means of gas burners, (see, for example, US. Pat. No. 3,484,225 and the'said German Patent Specification No. 397,427), or one of the pressing elements is cooled, usually with a liquid or gaseous cooling agent (see the said German Patent Specification No. 1,292,803 and the said published German Patent application No. 1,924,391, respectively), while I the other pressing elements is or is not heated or both pressing elements are cooled.

For reasons to be mentioned hereinafter, it is of very great importance in many manufacturing processes for the pressing surfaces of the pressing elements to have the same temperature throughout their surface. This is not the case in the known pressing devices due to the supply and dissipation, respectively, of thermal energy which is not uniformly distributed over the pressing elements and pressing surfaces, and due to the strong dependence upon the location of the thermal losses to the atmosphere. At the edges of the pressing surfaces, the thermal losses normally are larger than in the center. Expensive and complicated constructions of the pressing elements in the known pressing devices are necessary to achieve the isothermal character of the pressing surfaces (see, for example, the above German Patent specification No. 1,292,803). The problem is larger according as the shape of the article to be pressed and hence the shapes of the pressing surfaces are more complicated. Even with such complicated and expensive constructions of the pressing elements, known products do not have pressing surfaces which are isothermal throughout their surfaces during operation. Non-isothermal pressing surfaces result in pressed articles having an inhomogeneous temperature distribution. Upon cooling such articles to room temperature,

an uneven shrinkage occurs and material stresses are formed in said articles.

On the one hand, the articles often no longer meet the imposed requirements as regards shape which are usually stringent, which means a high reject percentage or may necessitate and expensive finish, for example,

repressing the inner surface of colour television display screens described in the said US Patent No. 3,484,225. On the other hand the material stresses may give rise directly to fracture, which also means a high.

reject percentage, or they manifest themselves later, for example, in the form of implosion danger in evacuated hollow glass articles. These latter problems generally necessitate time-consuming and expensive checks of the articles for mechanical stresses.

It is the object of the present invention to provide a pressing device of a simple and cheap construction, in which the said drawbacks are avoided.

In order to realize the end in view, the pressing device according to the invention is characterized in that the pressing elements are hollow and closed and the pressing surface of each pressing element constitutes a first heat transmission wall, each pressing element having elsewhere at least one second heat transmission wall, the cavity within each pressing element comprising a heat transporting medium which absorbs thermal energy from the first heat transmission wall while changing from the liquid phase into the vapour phase and delivers thermal energy' to the second heat transmission wall while changing from the vapour phase into the liquid phase, respectively absorbs thermal energy from the second heat transmission wall while changing from the liquid phase into the vapor phase and delivers thermal energy to the first heat transmission wall while changing from the vapor phase into the liquid phase, the cavity furthermore comprising a porous mass which connects the first to the second heat transmission wall in such manner that heat transporting medium condensed on the second or first heat transmission wall can flow back through said mass, by capillary action, to the first and the second heat transmission wall, respectively. I

If during operation of the device, thermal energy should be dissipated from one or both pressing surfaces, liquid heat transporting medium in the cavity of the relevant pressing element evaporates near the first heat transmission wall, flows in the vapor phase to the second heat transmission'wall as a result of the lower vapor pressure prevailing there owing to the slightly lower temperature at that area. The vapour then condenses on the second heat transmission wall while giving off the heat of evaporation to said wall, after which the condensate is returned, via the porous mass, by capillary action, while using the surface tension of the condensate, to the first heat transmission wall to be evaporated again there.

Since in places where the first heat transmission wall has a higher temperature a larger quantity of liquid heat transporting medium evaportates with consequently a larger heat dissipation, and in places where a lower temperature prevails a smaller evaporation of liquid heat transporting medium occurs and consequently a smaller heat dissipation, locally different temperatures will immediately be compensated for. The

first heat transmission wall, namely the pressing surface, therefor has the same temperature everywhere and thus is fully isothermal.

The thermal energy given off to the second heat transmission wall by condensation of heat transporting medium on said wall, can be given off to the atmosphere when the area of the said wall is sufficiently large, or can be further dissipated, for example, by means of a cooling agent. 7

Conversely, when during operation of the device thermal energy should be supplied to one or both pressing surfaces, liquid heat transporting medium in the cavity within the relevant pressing element takes up thermal energy at the area of the second heat transmission wall through said wall from a heat source, for example, an electric element, evaporates there and moves in the vapor phase to the first heat transmission wall as a result of the lower vapor pressure prevailing there owing to the slightly lower temperature at the area. The vapour then condenses on the first heat transmission wall while giving off the heat of evaporation through the first heat transmission wall to the pressing space, after which the condensate is returned again, by capillary action, via the porous mass to the second heat transmission wall to be evaporated again there.

In this case the largest quantity of vapour always condenses there on the first heat transmission wall where the lowest vapor pressure and hence the lowest temperature prevails and the smallest quantity of vapor condenses there where the highest temperature occurs. In this case also, locally different temperatures are thus immediately compensated for, so that the first heat transmission wall, namely the pressing surface, has the same temperature everywhere in this case also and therefore is fully isothermal.

Owing to the comparatively high heat of evaporation of liquids, a large quantity of thermal energy can be stored in the vapor and be transported per unit of time from one heat transmission wall to the other while a good heat transfer between the liquid and the heat transmission walls is ensured by condensation.

As a result of the large heat dissipation capacity of the heat transporting medium, it is possible in the lastmentioned case, in which thermal energy is supplied to the second heat transmission wall by a heat source, to choose the area of said wall to be small. This presents the advantage that only one compact heating element, for example a heating coil or a gas burner, is sufficient at the area of the second heat transmission wall to heat the whole first heat transmission wall at a unifrom temperature. The last-mentioned wall may have large dimensions exactly owing to the large heat transporting capacity of the medium.

Also as a result of the already mentioned heat transporting capacity of the medium in the cavity within the pressing element, substantially no temperature drop occurs during operation of the device between the two heat transmission walls, so that said walls have substantially the same temperature. This presents simple possibilities to measure the temperature of the pressing surface, the first heat transmission wall, during operation of the device. It is sufficient to use only one temperature sensor which may be arranged at any location of the second heat transmission wall. Also only one temperature control device will be sufficient.

As a result of the presence of the porous mass, the return of condensate from one heat transmission wall to the other heat transmission wall is ensured in all circumstances, so even return against gravity or without the effect of gravity. The pressing device is therefore independent of location which provides a great freedom in arrangement.

The porous mass may be constituted, for example, by ceramic materials, by sintered metal powders, by gauzes of glass fibres, of wireor plate-shaped material of metals or metal alloys, or by an arrangement of pipes or rods. A system of grooves in the wall of the cavity, whether or not in combination with one of the other above alternatives, is also among the possibilities.

The choice of the heat transporting medium is in the first instance determined by the operating temperature of the pressing surfaces. When said temperature is in the range from 600 to l500C, sodium, for example, may be chosen. Furthermore are to be considered as heat transporting media, for example, the metals potassium, lithium, cadmium, indium, lead, silver, metal salts such as the metal halogens zinc-chloride, aluminium bromide, cadmium iodide, calcium iodide, zinc bromide or mixtures thereof, nitrates and nitrites or mixtures thereof.

When the temperatures of the pressing surfaces are comparatively low, water, for example, is to be considered.

The porous mass connecting the first heat transmission wall to the second may cover a larger or smaller part of the wall surface area of the cavity within the pressing element. In a favorable embodiment of the pressing device according to the invention, however, the porous mass covers the whole surface of the cavity within the pressing element. This presents a few advantages. Since in the present case the porous mass fully covers the two heat transmission walls, the heat trans mission wall on which evaporation of liquid heat transporting medium takes place will be uniformly wetted by liquid. This reduces the possibility of local dry-boiling and consequently local overheating of said wall and stimulates that the said wall has a uniform temperature.

At the area of the heat transmission wall on which condensation of medium vapor takes place, the porous mass on said wall ensures that no formation of drops of condensate occurs under the influence of gravity. This furthers a uniform temperature of the side of the heat transmission wall remote from the porous mass. Formation of drops on one side of the heat transmission wall actually results in a locally higher thermal resistance which results in a different temperature on the other side of said wall.

In pressing devices in which high pressing forces are used, the first heat transmission walls serving as pressing surfaces are subject, during pressing, to a large mechanical load, the more so when said walls have large areas. In a number of cases the pressing surfaces are loaded also in the non-pressing condition. In order that the evaporation-condensation process of the heat transporting medium in the cavity within the pressing element can run off smoothly, said cavity is preferably evacuated. Dependent upon the heat transporting medium chosen, it may occur that the vapour pressure of the heat transporting medium in the cavity is below atmospheric pressure, not only at room temperature but also at the operating temperature of the pressing surfaces of the pressing elements which usually is high. If, for example, sodium is used at a heat transporting medium in the evacuated cavity, the sodium vapour pressure at 800K is 8 Torr 1 Torr 1 mm mercury pressure) and at l,l0OK it is 450 Torr. This means that also during the period in which there is no pressing, pressure forces are exerted on the pressing surfaces by the atmosphere.

A choice of thicker and hence more rigid first heat transmission walls, however, is not always possible because said walls are restricted to certain limits as regards thickness in connection with the thermal resistance.

In particular at high operating temperatures of the first heat transmission walls (pressing surfaces), at which temperatures the rigidity of said walls is considerably lower than at room temperature, all this may result in deformation (sagging) and tearing, respectively, of the said walls and implosion danger.

The porous mass may work loose from the wall and- /or capillary structure may be damaged so that it is no longer useful for the return of condensate.

In order to avoid the said drawbacks in a simple and cheap manner, a favorable embodiment of the pressing device according to the invention is characterized in that one or more supporting elements are arranged in the cavity for supporting the walls of the cavity against pressure forces exerted thereon from without said supporting elements permitting flow of medium vapor in the direction of heat transport.

Since the walls are supported now, they maintain their original shape and tearing of the first heat transmission wall, implosion, or damage to the capillary structure of the porous mass is prevented. Should normally the possibility exist that as a result of thermal stresses between the walls of the cavity and the porous mass, or by shoks or vibrations, said porous mass should work loose from the walls, the supporting elements now ensure also that the porous mass remains in its place.

The supporting elements may be constituted, for example, by perforated metal plate, which may or may not be connected together, by metal gauzes folded in a zig-zag manner or by a structure of rods or pipes.

In a favorable embodiment of the pressing device according to the invention the supporting elements are constituted by a compressed porous filling mass the pores of which have such a size that the relationship: (2. 7cos0/R)Appgh (2'ycos0 /R,) is fulfilled, wherein I 'y surface tension of liquid heat transporting medium.

R hydraulic radius of the pores in the porous mass.

0 angle of contact of liquid heat transporting medium in the pores of the porous mass.

R hydraulic radius of th pores in the filling mass.

6 angle of contact of liquid heat transporting medium in the pores of the'filling mass.

Ap pressure loss of liquid heat transporting medium in the porous mass between the first and the second heat transmission wall as a result of the resistance to flow of the said mass.

8 density of liquid heat transporting medium g acceleration of gravity.

h defference in height between the first and second heat transmission wall to be overcome by liquid heat transporting medium.

The cavity can be filled in a simple and cheap manner with such a filling mass. The filling mass may consist,

for example, of wires or tapes which are provided in bulk in the cavity and are then compressed, or the compression whether or not followed by sintering may be carried out previously.

The left-hand term of the above relationship denotes the resulting capillary force on the liquid heat transporting medium in the porous mass, where the hydraulic radius R is defined as 2. (surface/circumference) of the pores.

The angle of contact 0, being the angle between the liquid surface and the wall of the pore, depends for a given liquid upon the material of the wall of the pore and the nature of the wall surface. If in the present case, the material of the porous filling mass differs from that of the porous mass, the capillary rise may be mutually different with the same hydraulic radius.

By ensuring that the above-mentioned relationship is fulfilled, the porous mass will have a so much larger sucking effect for liquid than the filling mass, that at the area of the heat transmission wall on which condensation takes place, all the condensate is absorbed by the porous mass and nothing by the filling mass, while also in the direction of flow of the condensate, no condensate will pass from the porous mass to the filling mass on its way.

Transport of vapor from one heat transmission wall to the other through the filling mass therefore takes place substantially unhindered.

In practice all this means that the pores of the filling mass have a larger hydraulic radius than the pores of the porous mass. comparatively large dimensions of the pores of the filling mass are also desirable to minimize the flow losses of the vapor and hence the temperature gradient between the two heat transmission walls. The latter is of importance for the above-mentioned temperature measurement of the pressing surface.

According to the invention, steel wool is preferably used for the material for the filling mass.

Steel wool presents the advantage of a low price, can easily be compressed in all kinds of shapes and, in the compressed condition, can receive considerable surface pressures.

In the present pressing device it is always ensured that the first heat transmission walls serving as pressing surfaces are isothermal throughout their surface during operation. In circumstances, however, it may occur that in pressing device in which during pressing thermal energy originating from the material to be pressed should be removed from the pressing surfaces, said material is supplied to the pressing space with a higher or lower temperature than corresponds with the desirable temperature of pressing. Consequently the first heat transmission walls serving as pressing surfaces would also assume a higher or lower temperature. In order to be able to maintain the desirable temperaturefor the pressing process and the temperature level of the pressing surfaces, a larger quantity of thermal energy should be removed from the pressing space in the first case, and the heat dissipation should be restricted in the second case. In order to realize this, in a pressing device, in which during operation thermal energy is withdrawn from the pressing surfaces, each pressing element is provided, according to the invention, with a container comprising a control gas and being in open communication, at the area of the second heat transmission wall, with the cavity inside the pressing element in such manner that the control gas releases the second heat transmission wall more and less, respectively, when the medium vapour pressure exceeds the nominal value of said pressure corresponding to the nominal operating temperature of the first heat transmission wall, and falls below said nominal value, respectively.

It is achieved in this manner that when the temperature of the pressing space and hence the temperature of the pressing surface increases, a larger part of the heat transmitting surface of the second heat transmission wall is released by the poorly heat-conducting control gas so that increased heat dissipation occurs, while, when said temperature decreases, the available heat transmitting surface of the second heat transmission wall is reduced by the control gas, so that the heat dissipation is reduced and less heat disappears from the pressing space. The temperature of the pressing surfaces then remains substantially constant.

The operation is based on the fact that a comparatively small temperature variation of the pressing surface results in a comparatively large vapor pressure variation of the heat transporting medium in the cavity inside the pressing element. When the vapour pressure of said medium increases, the control gas is forced in the direction of the container, so that a larger surface of the second heat transmission wall is available for heat dissipation, while the reverse is the case when the vapor pressure decreases. The container may be arranged outside the pressing element. In the case of a pressing element having large dimensions with a large volume of the cavity inside it, the container may also be accommodated in the cavity, subject to it being arranged at a sufficiently large distance from the pressing surface and being preferably made from a heat insulating material, so as to prevent temperature variations of the pressing surface from being transmitted immediately to the control gas in the container. Otherwise, the pressure of the control gas immediately increases and decreases, respectively, as a result of said temperature variations, so that the control is disturbed or fully nullifled.

When the pressing device is not in operation, the control gas will spread inside the cavity. This does not matter, however, since, as soon as the device is put into operation, liquid heat transporting medium will evaporate near the pressing surface (the first heat transmission wall) and flow to the second heat transmission wall. The control gas is automatically driven back to the container by the flow of vapor.

In a favorable embodiment of the above pressing device, the container comprises a further porous mass in such manner that liquid heat transporting medium which has reached the container can flow back to the cavity by capillary action. This presents the advantage that liquid medium which has reached the container either by the effect of gravity or by condensation on a wall of said container, does not remain behind in the said container. The entire quantity of medium thus remains available for the evaporation-condensation process so that the full heat transporting capacity is maintained and there is no danger of dry-boiling and overheating of the pressing surface.

In order that the invention may be readily carried into effect, a few examples of the pressing device ac cording to the invention will now be described in greater detail, by way of example, with reference to the diagrammatic drawings which are not drawn to scale.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. la and 1b show pressing devices having two hollow and closed dies as pressing elements.

FIG. 2 shows a pressing device having two hollow and closed matrices as pressing elements.

FIG. 3 shows a pressing device having as pressing elements a combination of a hollow and closed die and a hollow and closed matrix.

FIG. 4 shows a pressing device having a hollow die and a hollow matrix, in which the cavity inside the die is in open communication with a container inside the die in which an inert non-condensable control gas is present and in which the cavity inside the matrix is also in open communication with a container comprising an inert, non-condensable control gas, which is arranged outside the matrix.

FIG. 5 shows a pressing device comprising two hollow and closed matrices of which each cavity is in open communication with a container comprising an inert non-condensable control gas.

FIG. 6 shows a pressing device having hollow and closed dies, the cavities of which are in open communication with containers arranged inside the dies and comprising an inert and non-condensable control gas.

DESCRIPTION OF THE PREFERRED EMBODIMENT Reference numeral 1 in FIG. 1a denotes a hollow and closed die having on the one hand a first heat transmission wall 2 serving as a pressing surface and on the other hand a second heat transmission wall 3. The walls of the cavity 4 are covered with a porous mass 5 which has a capillary structure. The cavity 4 furthermore comprises a suitably chosen quantity of sodium as a heat-transporting medium, and is otherwise evacuated. By means of an electric heating wire 6 wound around the second heat transmission wall 3, which wire is insulated thermally relative to the atmosphere by means of insulation material 7, thermal energy can be supplied to the cavity 4 through the second heat transmission wall 3.

The device furthermore comprises a hollow and closed die 8 which at one end comprises a first heat transmission wall 9 serving as a pressing surface, and on the other hand having a second heat transmission wall 10. The walls of the cavity 11 inside the die 8 are covered with a porous mass 12 which has a capillary structure. Cavity 11 also comprises a suitably chosen quantity of sodium as a heat transporting medium and is otherwise evacuated.

In this case also, an electric heating wire denoted by reference numeral 13 is wound around the second heat transmission wall 10 and is insulated thermally relative to the atmosphere by means of insulation material 14 and by means of which thermal energy can be supplied to the cavity 11 through the said heat transmission wall 10.

The dies 1 and 8 are relatively movable in the axial direction, being the direction of pressing, by means of hydraulic jacks l5 and 16, respectively, connected thereto. The first heat transmission walls 3 and 9, constituting pressing surfaces, can cooperate and compress material, for example a mass of soft glass, which is supplied to a pressing space 17 bounded by the said heat transmission walls.

The assembly of dies and hydraulic jacks is accommodated in a frame 18 which furthermore comprises a number of guide elements 19 which serve to guide the dies 1 and 8.

The operation of the pressing device is as follows. During operation, liquid sodium evaporates in cavity 4 of the die 1 at the area of the second heat transmission wall 3 by the absorption of thermal energy originating from the electric heating wire 6, through the said heat transmission wall. The sodium moves in the vapor phase through the cavity 4 to the first heat transmission wall 2, as a result of the lower vapor pressure prevailing near said wall due to the slightly lower temperature at that area. The sodium vapour then condenses on-the first heat transmission wall 2, while giving off the heat of evaporation to said wall, after which the condensate is returned, via the porous. mass 5, by capillary action, while using the surface tension of the condensate, to the second heat transmission wall 3 to be evaporated again there. The return of condensate takes place irrespective of the positon of the die, so also against gravity as in the case in the position shown. The first heat transmission wall 2 assumes the same temperature throughout its surface. Actually, the sodium vapor condenses always there where the lowest vapour pressure prevails, so that a locally different temperature is immediately corrected. So in this case we have a completely isothermal heat transmission wall 2. The liquid sodium in cavity 11 of the die 8 also evaporates at the area of the second heat transmission wall 10 by taking up thermal energy through said wall, which energy originates from the electric heating wire 13. The sodium moves in the vapor phase through the cavity 11 to the first heat transmission wall 9, as a result of the lower vapor pressure prevailing near said wall due to the slightly lower temperature at that region. The sodium vapor then condenses on the first heat transmission wall 9, while giving off the heat of evaporation to said wall, after which the condensate flows back, via the porous mass 12 and this time added by gravity, to

the second heat transmission wall 10 to be evaporated.

again there. In this case also the first heat transmission wall 9 is fully isothermal since local temperature differences are immediately compensated for by more or less condensation of sodium vapor, according as the temperature and the vapour pressure is lower or higher.

Since the two first heat transmission walls 2 and 9, being the pressing surfaces, are fully isothermal and it can also be ensured by a control of the heat supply by means of the electric heating wires 6 and 13 that the two pressing surfaces have the same temperature, the material compressed to an article in the pressing space 17 will have a homogeneous heat distribution.

When the article is cooled to room temperature, even shrinkage therefore occurs, so that substantially no material stresses occur in the article and the article maintains the originally given shape with all the advantages involved as described in the preamble.

The second heat transmission walls 3 and 10 have considerably smaller areas than the first heat transmission walls 2 and 9. The electric heating wires 6 and 13 consequently have compact dimensions. This is possible owing to the large heat transporting capacity of sodium vapor as compared with that of liquids and solid metals.

When high pressures are used, the first heat transmission walls 2 and 9 are subject to a high mechanical load. The cavities 4 and 1 1 are further evacuated in order that the evaporation-condensation process of the sodium can run off smoothly. The vapor pressure of the sodium both at room temperature and at an operating temperature of the first heat transmission walls 2 and 9 of, for example, 6003C, is much lower than 1 atmosphere, so that also when the pressing elements are not in the pressing condition, the first heat transmission walls are subject to a mechanical load.

In the device shown in FIG. lb, a porous filling mass 20 is present in cavity 4 and a porous mass 21 is present in cavity 11. For the rest, the device shown in FIG. lb is the same as that shown in FIG. la and the same reference numerals are used. The porous filling masses 21 and 22 serve as supporting elements having a high resistivity to pressure and receiving the pressure forces exerted on the first heat transmission walls 2 and 9, respectively, and they ensure that the said first heat transmission walls do not bend inwardly, crack and implode, respectively, or damage porous masses 5 and 12, respectively, as a result of which the capillary structure of the last-mentioned masses is no longer useful. The porous filling masses 20 and 21 also ensure that the porous masses 5 and 12 do not work loose from the walls and do not move away from the said walls.

The porous filling masses 20 and 21 consist of compressed steel wool the structure of which is coarser than that of the porous masses 5 and 12, respectively, that is to say the pores within the filling mass 20 have larger passages than the pores in the mass 5, and the pores within the filling mass 21 have larger passages than the pores in the mass 12. Consequently, all sodium condensed on thefirst heat transmission wall 2 is drawn into the pores of the porous mass 5 and nothing is drawn into the pores of the porous filling mass 20. Return of condensate to the second heat transmission wall 3 therefore takes place only through porous mass 5, so that all the pores in the filling mass 20 remain available for the transport of sodium vapor from the second heat transmission wall 3 to the first heat transmission wall 2. Analogously in the case of the disc 8, transport of sodium condensate from the first heat transmission wall 9 to the second heat transmission wall 10 takes place only through porous mass 12 and transport of sodium vapour from the second to the first heat transmission wall occurs through the porous filling mass 21.

Reference numeral 25 in FIG. 2 denotes a hollow and closed matrix having a first heat transmission wall 26 serving as a pressing surface, and in addition a second heat transmission wall 27. Cavity 28 inside the matrix 25 again comprises a suitably chosen quantity of heat transporting medium and is otherwise evacuated. The walls of the cavity 28 are lined with a porous mass 29 which has a capillary structure.

At the area of the second heat transmission wall 27, a number of ducts 30 is present through which a cooling agent can be conveyed. The matrix 25 is connected via a plunger 31 to a construction not shown which can reciprocate the matrix in the direction of pressing and supplies the pressure proper.

A matrix 33 which is hollow and closed is arranged on a pressure stand 32 and comprises on the one hand a first heat transmission wall 34 serving 6ans, an: as a pressing surface, and on the other hand a second heat transmissionwall 35. The cavity 36 of matrix 33 also comprises a suitably chosen quantity of heat transporting medium, while said cavity is otherwise evacuated. The walls with the cavity 36 are lined with a porous mass 37 which has a capillary structure.

At the region of the second heat transmission wall 35, a number of ducts 38 are present through which a cooling agent can be conveyed. First heat transmission walls 26 and 34, being the pressing surfaces, are matched to each other, as regards design, so that they can give the material to be compressed in pressing space 39 the desirable shape. The operation of this pressing device is as follows:

Material to be compressed of comparatively high temperature is supplied to the pressing space 39 and is compressed by lowering the matrix 25 and is shaped by said matrix and by matrix 33.

First heat transmission walls 26 and 34 assume a certain high temperature. As a result of this, liquid heat transporting medium in the cavity 28 evaporates at the area of the first heat transmission wall 26, flows in the vapor phase to the second heat transmission wall 27, due to the lower vapour pressure there as a result of the lower temperature at the area, which is maintained in that a cooling agent is conducted through ducts 30. The vapor then condenses on the second heat transmission wall 27 while giving off the heat of evaporation to said wall, after which the condensate flows back through the porous mass 29 by capillary action, and in this case also aided by gravity, to the first heat transmission wall 26 to be evapoarted again there.

The first heat transmission wall 26 is again isothermal throughout its surface. The porous mass 29 actually ensures that the first heat transmission wall 26 is uniformly wetted while at the area where the temperature is locally above the desirable operating temperature of the first heat transmission wall 26, more liquid heat transporting medium evaporates and less evaporation occurs at the area where the temperature is locally lower. Temperature differences are thus compensated for immediately. Inside the cavity 36 of the matrix 33 an evaporation-condensation process occurs in an analogous manner as in the matrix 25, so that this need not be described here. In this case, the second heat transmission wall 35 is cooled by conducting a cooling agent through ducts 38. Return of condensate from the second heat transmission wall 35 to the first heat transmission wall 34 occurs through the porous mass 37 by capillary action, inthis case against gravity. For the same reasons as for the first heat transmission wall 26, the first heat transmission wall 34 is also isothermal throughout its surface. If desirable, supporting elements for receiving the pressure forces exerted on the matrix walls may also be arranged in the cavity 28 and- /or 36 of the present pressing device.

In the pressing device shown in FIG. 3 are present hollow and closed die 45 and a hollow and closed matrix 46. For corresponding components as in-FIG. 2, the same reference numerals are used.

At the area of the second heat transmission wall 27, the die 45 comprises a helical cooling pipe 47, through which a cooling agent can be conducted.

In order to maintain, during operation, the first heat transmission wall 34 of the matrix 46 at the desirable temperature, a burner 48 is arranged at the area of the second heat transmission wall 35, by means of which thermal energy can be supplied to the heat transporting medium in the cavity 36, through the last-mentioned wall.

During operation of this pressing device, warm material to be compressed is supplied to the pressing space 39 within the first heat transmission wall 34, and compressed by the die 26.

An evaporation-condensation process occurs within the cavity 36 of the matrix 46, in which liquid heat transporting medium evaporates at the area of the second heat transmission wall 35, condenses on the first heat transmission wall 34, while giving off the heat of evaporation to the said wall, and in which the condensate then flows back to the second heat transmission wall 35 via porous mass 37. The evaporationcondensation process ensures in a similar manner as in the pressing device shown in FIGS. la and 1b that the first heat transmission wall 34 is entirely isothermal at the desirable temperature.

An evaporation-condensation process occurs in the cavity 28 of the die 45, in which the first heat transmission wall 26 is cooled. For that purpose, liquid heat transporting medium evaporates at the area of the said wall, flows in the vapor phase to the second heat transmission wall 27, and condenses on said wall while giving off the heat of evaporation. Thermal energy is removed from the second heat transmission wall 27 by means of a cooling agent conducted through helical cooling pipe 47. In a manner analogous to that of the pressing device shown in FIG. 2, the first heat transmission wall 21 is fully isothermal.

By a suitable arrangement of the burner 48 and of the cooling capacity of the helical cooling pipe 47 it is achieved that the two first heat transmission walls 34 and 26 have the same temperature.

If desirable, supporting elements may be arranged in this case also in the cavities 28 and 36.

The pressing device shown in FIG. 4 is in general equal to that shown in FIG. 2. For corresponding components the same reference numerals are therefore used. In the present case, helical cooling pipes 50 and 52, respectively, are arranged around the second heat transmission walls 27 and 35. Furthermore, at the area of the second heat transmission wall 27, the cavity 28 of matrix 25 is in open communication with a container 52 which in this case is constituted by a space inside the plunger 31. The container 52 comprises an inert, noncondensable and substantially heat-insulating control gas which can be supplied to an withdrawn from, respectively, the said container via a duct 53 in which a cock 54 is incorporated. The cavity 36 of matrix 33 is also in open communication with a container 55 which contains an inert and non-condensable heat-insulating control gas and which communicates with said cavity at the region of the second heat transmission wall 35. The container 55 comprises an inlet and an outlet 56 in which a cock 57 is incorporated.

When, during operation of said pressing device, in which the first heat transmission walls 26 and 34 are again fully isothermal for the above-mentioned reasons, the temperature level of said walls should vary, for example, because the material supplied to the pressing space 39 does not always have the same temperature, the control gas in the two containers 52 and 55 ensures that the deviations from the desirable temperature level of the said walls are nullified. This is effected as follows.

During operation of the matrix 25, all the control gas is present in the container 52, since, due to the evaporation-condensation process in the cavity 28, also the control gas is driven in the direction from the first to the second heat transmission wall by the vapor of heat transporting medium. A sharp interface is formed between the vapor and the control gas. By filling container 52 with the correct quantity of control gas it is ensured that at the nominal operating temperature of the first heat transmission wall 26, said interface is located in such a place that the second heat transmission wall 27 is partly covered with control gas. This place is indicated by a broken line.

When the temperature of the isothermal first heat transmission wall 26 increases, more liquid heat transporting medium evaporates near said wall. As a result of this the vapor pressure in the cavity 28 increases which results in the control gas being forced further into the container 52. So the interface moves upwards, as a result of which a larger part of the surface of the second heat transmission wall 27 is set free and thus more thermal energy is removed through said wall from the cavity 28. This has a temperature stabilizing effect.

Conversely, when the temperature of the isothermal first heat transmission wall 26 decreases, a smaller quantity of liquid heat transporting medium evaporates near the said wall so that the vapour pressure in cavity 28 decreases. The interface vapour-control gas moves downwards, so that a larger part of the surface of the second heat transmission wall 27 is blocked by the control gas and hence a smaller surface is available for heat dissipation. So, a smaller amount of thermal energy is dissipated. This also has a temperature-stabilizing effect. In controlling the heat transmission surface of the second heat transmission wall by means of the control gas, the fact is used that a comparatively small temperature variation of the isothermal first heat transmission wall results in a comparatively large vapour pressure variation of the heat-transporting medium. All this has for its result that isothermal first heat transmission wall 26 always remains. at the same temperature.

In the case of matrix 33, the control of the heat transmission surface of the second heat transmission wall 35, in the case of variation of the temperature of the isothermal first heat transmission wall 34, occurs in an identical manner so that this need not be described. The place of the interface control gas vapour near the second heat transmission wall 35, corresponding to the nominal operating temperature of the isothermal first heat transmission wall 34, is denoted by a broken line in this case also.

The pressing device shown in FIG. corresponds in general to that shown in FIG. 4. Therefore, the same reference numerals are used for corresponding components.

In this case, the two second heat transmission walls 27 and 35 are cooled by spraying the said walls with a cooling agent, for example water, which is denoted by arrows. The containers 52 and 55 containing control gas are arranged slightly differently in this case.

In this pressing device the walls of the containers 52 and 55. are also lined with porous masses 29 and 37, respectively. This presents the advantage-that when liquid heat transporting medium should get in the containers, for example by condensation of medium vapour on a wall of a container, or by flowing in of liquid under the influence of gravity, it is ensured in all circumstances that the said heat transporting medium is returned to cavities 28 and 36, respectively, by capillary action through the porous mass. In this manner all the heat transporting medium remains available for taking part in the evaporation-condensation process. Thus the full heat transporting capacity of the medium is maintained, while overheating of the-first heat transmission wall by dry-boiling due to an insufiicient amount of heat transporting medium being present, cannot occur. For the rest, the operation of this pressing device is the same as that shown in FIG. 4, so that it is not described here.

The pressing device shown in FIG. 6 greatly corresponds to the device shown in FIG. 1, so that for corresponding components the same reference numerals are used. In the present case, however, thermal energy is removed, via an evaporation-condensation process in the cavities 4 and 11, from the first heat transmission walls 2 and 9 instead of being supplied as in the device as shown in FIG. 1. For that purpose, helical cooling pipes 60 and 61, respectively, through which a cooling agent can be conducted, are arranged around the second heat transmission walls 3 and 10.

In the present device, containers 62 and 63 containing control gas are arranged inside cavity 4 of die 1 and cavity 11 of die 8, respectively, at the area of the second heat transmission walls 3 and 10, respectively. Via

apertures 64, container 62 is in open communication with cavity 4 and container 63 is in open communication with cavity 11 via apertures 65.

Container 62 can be filled with control gas or be emptied via ducts 66 and 67 while in the case of container 63, this may be done via ducts 68 and 69.

The inner walls of the container 62 are lined with a porous mass 70 which adjoins the porous mass 5 on the walls of the cavity 4. The inner walls of the container 63 are also lined with a porous mass 71 which adjoins porous mass 12 on the walls of the cavity 11.

The operation of the pressing device corresponds to that of the pressing devices shown in FIGS. 4 and 5, with the exception of course of the fact that in the present case both dies are reciprocal so that this is not further described. The broken lines again denote the interfaces medium vapour-control gas for the situation in which the isothermal first heat transmission walls 2 and 9 have their nominal operating temperatures.

Containers 62 and 63 are made of heat insulating material and arranged at a comparatively large distance from the first heat transmission walls 2 and 9, respectively. As a result of this it is achieved that any variation of the temperature level of the relevant first heat transmission wall results in no variation of the control gas temperature, so that said control gas would vary in pressure, and the control of the heat transmission surface of the second heat transmission wall would be disturbed. Arrangement of the containers inside the dies 1 and 8 has of course the advantage of a compact construction.

Pourous mass 70 in the container 61 and porous mass 71 in the container 63 ensure in all circumstances that any liquid heat transporting medium which has got in the containers can flow back again to the cavities 4 and 1 1, respectively.

Of course all kinds of other embodiments of the pressing device are possible, in addition to those described, without departing from the scope of the present invention.

What is claimed is:

1. An apparatus for pressing a material and operable with a thermal energy source, comprising: two pressing elements, each comprising walls having an inner surface which define a closed cavity, said walls including a first heat transfer wall and spaced therefrom a second heat transfer wall, a heat transporting medium within each cavity, said medium being changeable from liquid to vapor when heat is transferred thereto, and from vapor to liquid when heat is removed therefrom, a porous mass on said inner wall surfaces interconnecting said first and second heat transfer walls, said porous mass permitting capillary flow of said medium in its liquid form, said first heat transfer walls having outer surfaces being pressing surfaces, means for moving at least one of said elements relative to the other such that said pressing surfaces are movable toward each other for pressing said material therebetween and away from each other, said first heat transfer walls being in heattransfer relationship with said material when in contact therewith, and said second heat-transfer walls being in heat transfer relationship with said thermal energy source whereby heat is transferrable from said thermal energy source through said second heat transfer walls to said heat transporting medium in liquid form which vaporizes and moves to said first heat transfer walls where said medium is condensed as heat is tranferred from the medium to and through the first heat transfer wall to said medium to be pressed, and said condensed medium flows by capillary action through said porous mass, back to said second heat transfer wall.

2. Apparatus according to claim 1 whrein the porous mass covers the whole inner surface of the cavity within the pressing element.

3. Apparatus according to claim 1 and subjectable to external pressure forces against and compressing the walls of said pressing elements, wherein each of said pressing elements further comprises at least one supporting element within the cavity contacting said inner surfaces for providing support against said pressure.

4. Apparatus according to claim 3 wherein each of said supporting elements comprises a compressed porous filling mass, the pores of which have such a size that the relationship (2 y cos 6/R) Ap pgh (2 7 cos (MR is fulfilled, in which 7 surface tension of liquid heat transporting medium.

R hydraulic radius of the pores in the porous mass.

angle of contact of liquid heat transporting medium in the pores of the porous mass.

R hydraulic radius of the pores in the filling mass.

0, angle of contact of liquid heat transporting medium in the pores of the filling mass.

Ap pressure loss of liquid heat transporting medium in the porous mass between the first and second heat transmission wall as a result of the resistance to flow of said mass.

6 density of liquid heat transporting medium.

g acceleration of gravity.

h difference in height between the first and the second heat transmission wall to be overcome by liquid heat transporting medium.

5. Apparatus according to claim 4 wherein said porous filling mass comprises compressed steel wool.

6. Apparatus according to claim 5 wherein each pressing element further comprises within the cavity a container having an internal space and including therein an inert, non-condensable control gas, the container being nearer to the second heat transfer wall than to the first and having low thermal conductivity with said first wall, said space being in open communication with the cavity within said pressing element, whereby said control gas exposes for heat transfer more of the second heat transmission wall when the medium vapor pressure exceeds the nominal value of the said pressure corresponding to the nominal operating temperature of the first heat transmission wall, and exposes less of said second wall when the vapor pressure falls below said nominal value.

7. A pressing device as claimed in claim 6, characterized in that the container comprises a further porous mass in such manner that liquid heat transporting medium which has reached the container can flow back, through said further mass, to the cavity by capillary action.

8. An apparatus for pressing a material and operable with a thermal energy sink comprising: two pressing elements, each comprising walls having an inner surface which define a closed cavity, said walls including a first heat transfer wall and spaced therefrom a second heat transfer wall, a heat transporting medium wtihin each cavity, said medium being changeable from liquid to vapor when heat is transferred thereto, and from vapor to liquid when heat is removed therefrom, a porous mass on said inner wall surfaces interconnecting said first and second heat transfer walls, said porous mass permitting capillary flow of said medium in its liquid form, said first heat transfer walls having outer surfaces being pressing surfaces, means for moving at least one of said elements relative to the other such that said pressing surfaces are movable toward each other for pressing said material therebetween and away from each other, said first heat transfer walls being in heattransfcr relationship with said material when in contact therewith, and said second heat-transfer walls being in heat transfer relationship with said thermal energy sink whereby heat is transferrable from said material through said first heat transfer walls to said heat transporting medium in liquid form which vaporizes and moves to said second heat transfer walls where said medium is condensed as heat is transferred from the medium to and through said second heat transfer walls to said heat sinks, and said condensed medium flows by capillary action through said porous mass, back to said first heat transfer walls. 

1. An apparatus for pressing a material and operable with a thermal energy source, comprising: two pressing elements, each comprising walls having an inner surface which define a closed cavity, said walls including a first heat transfer wall and spaced therefrom a second heat transfer wall, a heat transporting medium within each cavity, said medium being changeable from liquid to vapor when heat is transferred thereto, and from vapor to liquid when heat is removed therefrom, a porous mass on said inner wall surfaces interconnecting said first and second heat transfer walls, said porous mass permitting capillary flow of said medium in its liquid form, said first heat transfer walls having outer surfaces being pressing surfaces, means for moving at least one of said elements relative to the other such that said pressing surfaces are movable toward each other for pressing said material therebetween and away from each other, said first heat transfer walls being in heat-transfer relationship with said material when in contact therewith, and said second heat-transfer walls being in heat transfer relationship with said thermal energy source whereby heat is transferrable from said thermal energy source through said second heat transfer walls to said heat transporting medium in liquid form which vaporizes and moves to said first heat transfer walls where said medium is condensed as heat is tranferred from the medium to and through the first heat transfer wall to said medium to be pressed, and said condensed medium flows by capillary action through said porous mass, back to said second heat transfer wall.
 2. Apparatus according to claim 1 whrein the porous mass covers the whole inner surface of the cavity within the pressing element.
 3. Apparatus according to claim 1 and subjectable to external pressure forces against and compressing the walls of said pressing elements, wherein each of said pressing elements further comprises at least one supporting element within the cavity contacting said inner surfaces for providing support against said pressure.
 4. Apparatus according to claim 3 wherein each of said supporting elements comprises a compressed porous filling mass, the pores of which have such a size that the relationship (2 gamma cos theta /R) -p - Rho gh (2 gamma cos 1/R1) is fulfilled, in which gamma surface tension of liquid heat transporting medium. R hydraulic radius of the pores in the porous mass. theta angle of contAct of liquid heat transporting medium in the pores of the porous mass. R1 hydraulic radius of the pores in the filling mass. 1 angle of contact of liquid heat transporting medium in the pores of the filling mass. Delta p pressure loss of liquid heat transporting medium in the porous mass between the first and second heat transmission wall as a result of the resistance to flow of said mass. delta density of liquid heat transporting medium. g acceleration of gravity. h difference in height between the first and the second heat transmission wall to be overcome by liquid heat transporting medium.
 5. Apparatus according to claim 4 wherein said porous filling mass comprises compressed steel wool.
 6. Apparatus according to claim 5 wherein each pressing element further comprises within the cavity a container having an internal space and including therein an inert, non-condensable control gas, the container being nearer to the second heat transfer wall than to the first and having low thermal conductivity with said first wall, said space being in open communication with the cavity within said pressing element, whereby said control gas exposes for heat transfer more of the second heat transmission wall when the medium vapor pressure exceeds the nominal value of the said pressure corresponding to the nominal operating temperature of the first heat transmission wall, and exposes less of said second wall when the vapor pressure falls below said nominal value.
 7. A pressing device as claimed in claim 6, characterized in that the container comprises a further porous mass in such manner that liquid heat transporting medium which has reached the container can flow back, through said further mass, to the cavity by capillary action.
 8. An apparatus for pressing a material and operable with a thermal energy sink comprising: two pressing elements, each comprising walls having an inner surface which define a closed cavity, said walls including a first heat transfer wall and spaced therefrom a second heat transfer wall, a heat transporting medium wtihin each cavity, said medium being changeable from liquid to vapor when heat is transferred thereto, and from vapor to liquid when heat is removed therefrom, a porous mass on said inner wall surfaces interconnecting said first and second heat transfer walls, said porous mass permitting capillary flow of said medium in its liquid form, said first heat transfer walls having outer surfaces being pressing surfaces, means for moving at least one of said elements relative to the other such that said pressing surfaces are movable toward each other for pressing said material therebetween and away from each other, said first heat transfer walls being in heat-transfer relationship with said material when in contact therewith, and said second heat-transfer walls being in heat transfer relationship with said thermal energy sink whereby heat is transferrable from said material through said first heat transfer walls to said heat transporting medium in liquid form which vaporizes and moves to said second heat transfer walls where said medium is condensed as heat is transferred from the medium to and through said second heat transfer walls to said heat sinks, and said condensed medium flows by capillary action through said porous mass, back to said first heat transfer walls. 